U.S. application Ser. No. 06/874,041, filed June 13, 1986, now abandoned and refiled on Mar. 30, 1988, as continuation-in-part application Ser. No. 07/175,171 assigned to the assignee of the present invention, describes an asymmetrical run-length-limited (RLL) code suitable for optical storage. However, this code provides no coding gain. It requires two or more contiguous NRZ 1 bits, but there is no restraint on the number of NRZ 0 bits.
U.S. Pat. No. 4,413,251, also assigned to the assignee of the present invention, discloses a method for generating a sliding block (1,7) RLL code with a coding rate of 2/3. However, for (1+D) partial response channels, this method provides no coding gain; and for (1+D).sup.2 channels, this method provides only a 1.8 dB coding gain. This code is generated by a sequential scheme that maps two bits of unconstrained data into three bits of constrained data.
There is a need for coding techniques which provide coding gains of at least 3 dB unnormalized, and at least 2.2 dB when normalized, for input-restricted (1+D) and (1+D).sup.2 channels with respect to existing asymmetric RLL codes and the RLL (1,7) code, and which coding techniques are especially suitable for optical recording.